1. Field of the Invention
The invention relates generally to the field of determining particle size distributions, including volume distributions (particle volume per particle diameter interval); area distributions (particle area per particle diameter interval); and particle number distributions (particle number per particle diameter interval).
More particularly the invention relates to methods and apparatus for determining the size distribution of small particles using multiple light beams to extend the angular scatter measurement range of a particle size analyzer.
2. Description of the Related Art
Methods and apparatus are well known for determining the size distribution of particulate material for particles in the approximate range of 0.1 to 100 microns in diameter. For example, Wilcock, in U.S. Pat. No. 3,873,206, issued Mar. 25, 1975, and Wertheimer, in U.S. Pat. No. 4,134,679, issued Jan. 16, 1979, both assigned to the assignee of the present invention, describe such methods.
A further example of prior art for determining the size distribution of small particles is taught in U.S. Pat. No. 5,094,532, to Trainer et al., issued Mar. 10, 1992, also assigned to the assignee of the present invention. According to Trainer et al., a beam of light is directed into a scattering medium to measure the size distribution of particles experiencing Brownian motion.
In particular, according to Trainer et al., the frequency of the scattered light is compared with the frequency of the source light. The comparison results in the generation of a first signal having a frequency that varies with time and is indicative of the difference in the frequency between the scattered light and the source light. A second signal is generated having a magnitude which varies with frequency on a linear scale. The frequency scale is then translated to a logarithmic scale. Finally, the translated second signal is deconvolved to determine the size distribution of moving particles within the scattering medium.
Although scattered light frequency measurement is now a recognized technique for determining the size distribution of very small particles (less than 2 microns in diameter); most commercially available particle size distribution measuring systems (typically used to measure particles from 0.1 to 1000 microns in diameter), use the technique of analyzing the angular distribution of light scattered (from the particles) to determine particle size distribution. This is because as particle size increases the velocities of particles due to Brownian motion become too small to measure.
An example of a commercially available instrument that analyzes the angular distribution of light scattered from particles to determine particle size distribution is the Microtrac Standard Range Analyzer (SRA) manufactured by Leeds & Northrup Company ("Microtrac" is a registered trademark owned by Leeds & Northrup Company).
In the SRA a collimated monochromatic light beam irradiates an ensemble of particles flowing perpendicularly through the collimated beam. Light scattered from the particles emerges from the interaction volume at an angle from the axis of the collimated beam. The scattered light is collected by a lens placed in the path of the scattered light.
The scattered light pattern, focused in the focal plane of the lens, is typically measured by an array of photodetectors placed in the focal plane. The angular extent of the scatter pattern is determined by the size of the scattering particle. The smaller the particle, the wider the angular extent of the scatter; the larger the particle, the narrower the angular extent of scatter.
It is well known in the art that the scattered light from a mixture of different sized particles is simply the sum of all of the individual scattering distributions from each size, weighted by the total particle number at that size. The particle size distribution may be "inverted" from this composite scattering distribution by using well known mathematical algorithms, such as deconvolution.
In order to obtain good size resolution in the size distribution in a system that analyzes the angular distribution of light scattered (for example, the SRA), the scattered light bundles at each angle must be focused, with low optical aberration, to a small spot on the detector array. The scattering angle range of a single lens is limited by aberrations, such as distortion and field curvature, which increase with lens field angle.
For a typical system with a collector lens on the axis of the collimated beam (like the SRA), the angular range collected by the lens also depends upon the lens diameter and the distance from the sampling volume.
Because of the aforementioned factors, optical measuring systems like the SRA have an angular range limited to approximately 15 degrees. This results in a high resolution measurement capability extending down to only a few microns in particle diameter when using a 0.75 micron light source, typically a laser.
Those skilled in the art will recognize that extending the angular scatter measurement range of systems that analyze the angular distribution of light (to determine particle size distribution) has several benefits, including being able to increase the measurement capabilities of such systems to the submicron range; while at the same time being able to perform data collection in time frames that are typically shorter then would be required by the aforementioned light frequency measurement techniques.
In order to achieve these benefits, larger scattering angles have been measured using an off axis collector lens and a measurement photodiode detector array in the focal plane of the off axis lens. This arrangement has allowed the angular range to be extended out to about 50 degrees and a high resolution particle size determination to be extended down to approximately 0.4 microns.
An example of a commercially available instrument that analyzes the angular distribution of light using both an on axis and off axis collector lens (and their associated detector arrays), together with shared data processing equipment, is the Microtrac Full Range Analyzer (FRA) manufactured by Leeds & Northrup Company.
The further extension of the angular measurement range beyond the range achieved by using systems like the FRA, can be accomplished by adding more off axis collector lenses and photodetector arrays. However, this approach is problematic in that: (a) each lens and detector array set has to be aligned, calibrated and matched to the other lens/detector sets to measure a wider scatter pattern; and (b) since the lens angular field is limited by aberrations, many expensive lens/detector sets are needed to cover the large angular range required for measuring small particles using a single light source.
It should be noted that in addition to the aforementioned techniques for determining small particle size distributions, other techniques exist which make the desired determination utilizing (a) fringe patterns caused by the interference of at least two light beams or (b) particle counting techniques.
Exemplary fringe pattern oriented techniques are described in U.S. Pat. No. 4,179,218, to Erdmann et al., issued Dec. 18, 1979; U.S. Pat. No. 4,329,054, to Bachalo, issued May 11, 1982; U.S. Pat. No. 4,537,507, to Hess, issued Aug. 27, 1985; U.S. Pat. No. 4,596,036, to Norgren et al., issued Jun. 17, 1986; and U.S. Pat. No. 4,701,051, to Buchhave et al., issued Oct. 20, 1987.
Exemplary particle counting techniques are described in U.S. Pat. No. 4,251,733, to Hirleman, Jr., issued Feb. 17, 1981; U.S. Pat. No. 4,348,111, to Goulas et al., issued Sep. 7, 1982; U.S. Pat. No. 4,444,500, to Flinsenberg et al., issued Apr. 24, 1984; and U.S. Pat. No. 4,957,363, to Takeda et al., issued Sep. 8, 1990.
It should also be noted, however, that none of the patents referenced hereinabove teach, claim or even suggest utilizing multiple light sources, positioned at different angles and operated one at a time in sequence, to extend the angular scatter measurement range of an optical scattering particle size analyzer which is the subject of the present invention.